4 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
three or four pretty distinct ranges which enclose the basin, and which contribute 
to injure its climate; for there is little wood, and the winds from every quarter 
sweep unchecked over the extensive and bare plateaus of granite and basalt of 
which it is chiefly composed.* On the 25th May I was exposed to a piercing wind, 
having a temperature of 36°, with frequent showers of hail. A few days later I 
engaged a horse to carry me across the highest part of the range of the Cevennes, 
and the sources of the Loire, down to the valleys of the Ardéche. My kind friend 
M. Bertranp had given me a route which should embrace the most interesting 
geological points; and with his map as a guide, I started alone with a pony, to 
sleep at the foot of the Mont Mezenc, at a village bearing the unpromising name 
of Fay-le-froid. Though it was the last day but one of May, little appearance of 
spring was visible ; indeed, it had hardly an opportunity of making any impression 
on the singularly bare and rugged features of a country nearly destitute of trees, 
and often covered for miles with brown angular fragments of basalt. 
The position of Fay-le-froid, a meagre village of little more than a single 
street, is somewhat greener; it lies upon the northern slope of the Mont Mezenc, 
and near the church is a bed of basalt, containing fragments of trachyte and 
granite, with abundance of olivine. Early next morning I was on the gentle 
ascent which leads in two hours to the summit of the Mont Mezenc. A fog pre- 
vented me from enjoying the view,t but it afterwards cleared away sufficiently 
to enable me to examine the geological section below the point called La Croix 
de Boutizres, which is 800 feet vertically lower than the top of the Mezenc, and 
is situated a little to the south of it. Here the escarpment to the south-east forms 
a sort of imperfect amphitheatre, in which the Salliouse (a rivulet joining the 
Erieux, a tributary of the Rhone) takes its rise. This hollow, sometimes called 
Le Cirque de Clusels, presents a section which has justly obtained some celebrity 
amongst geologists. The peculiarity which it presents is the undoubted swper- 
position of trachyte and phonolite or clinkstone (which are felspathic lavas), to 
common basalt and vesicular scorize. The section which I obtained, and from 
which I took specimens, is shewn in Plate II., fig. 1. There is no proper super- 
position of trachyte to phonolite ; the latter appears to pass into the former, and 
sometimes to form veins ih it. I have no doubt that the trachyte is scorified and 
rendered vesicular and ochrey by the heat of the basalt injected from below. This 
appears to be the utmost which can be legitimately inferred from this section ; and 
it is so far a satisfactory conclusion, since it does not contradict the generally 
established view of the posteriority of basaltic to felspathic lavas, which rule be- 
sides receives, in this immediate neighbourhood, so unquestionable a support from 
* See Mr Scropz’s Panorama from the Montagne d’Ours. 
+ In clear weather Mont Blane is visible from hence. Bertrand, Description du Puy, &c., 
p- 124. 
